The increasing demand for ethylene as a chemical feedstock along with the limited availability of petroleum reserves and the emergence of shale gas production have fueled renewed interest in developing direct and efficient catalytic sequences for the oxidative coupling of methane (OCM) to ethylene. Numerous studies have focused on OCM with O2 as the oxidant to optimize the C2 yield and to elucidate the reaction mechanism. Alkaline earth oxides, early transition metal oxides, and rare earth oxides are found to be promising catalysts, and enhanced catalytic performance can be achieved by adding dopants and/or promoters. Nevertheless, large scale applications of OCM with O2 have yet to be implemented, primarily attributable to the large thermodynamic driving force for over-oxidation (FIG. 1), which compromises C2 selectivity, and the difficulty of reactor engineering for such highly exothermic processes. Other approaches for converting CH4 to valuable feedstocks such as the non-oxidative coupling of CH4 have attractions but also significant limitations, and to date have not seen large-scale use.
An alternative to the above approaches seeks to moderate the driving force for methane over-oxidation by employing a “soft” oxidant such as S2 (SOMC). Note that the ΔG for CH4 over-oxidation by S2 is −236 kJ/mol versus −1294 kJ/mol for O2 (FIG. 1), suggesting that higher ethylene selectivities/yields might be possible using less aggressive ‘soft’ oxidants along with an optimum catalyst. The lower SOCM exothermicity versus that with O2 might also offer advantages in reactor design, and the H2S co-product could be efficiently reconverted to sulfur via the Claus process. In earlier work it was envisioned that the optimum catalysts for creating S—C bonds along plausible sulfur-based OCM reaction coordinates might be those which cleave S—C bonds and are not poisoned by large quantities of S2, such as hydrodesulfurization catalysts.
Accordingly, a series of transition metal chalcogenides (e.g., TiS2, RuS2, MoS2, PdS, etc.) was investigated for SOCM in a specialized reactor, affording CH4 conversions and C2H4 selectivities at 950° C. of 6-9% and 4-9%, respectively. In conjunction with these experimental studies, computational investigations showed that the M-S bond strength has a major influence on the CH4 conversion and the C2H4 selectivity, the two of which are inversely related. However, issues remain in the art as to the role of the transition metal and the particular oxide support in this catalytic transformation, and as to whether there are alternatives to such a noble metal catalyst system—to better realize the benefits and advantages available through use of a S2 oxidant.